Re: [PATCH 2/2] drm/i915: do not setup backlight if not available according to VBT

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 09 Apr 2014, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Apr 09, 2014 at 01:35:35PM +0300, Jani Nikula wrote:
>> Some machines use an external EC for controlling the backlight. Info
>> about this is present in the VBT. Do not setup native backlight control
>> if no PWM backlight is available or supported according to VBT. The
>> acpi_backlight interface appears to work for the EC control.
>> 
>> In most cases there has been no harm done, but it looks like there are
>> machines out there that have both an EC and our PWM line connected to
>> the same wire. This, obviously, does not end well.
>> 
>> This should fix the regression caused by
>> commit bc0bb9fd1c7810407ab810d204bbaecb255fddde
>> Author: Jani Nikula <jani.nikula@intel.com>
>> Date:   Thu Nov 14 12:14:29 2013 +0200
>> 
>>     drm/i915: remove QUIRK_NO_PCH_PWM_ENABLE
>> 
>> AFAICT the quirk removed by the above commit effectively resulted in
>> i915 not driving the backlight PWM output, thus not messing things up.
>> 
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76276
>> Reference: https://bugzilla.kernel.org/show_bug.cgi?id=47941
>> CC: Aaron Lu <aaron.lu@intel.com>
>> CC: Kamal Mostafa <kamal@canonical.com>
>> CC: Eric Griffith <EGriffith92@gmail.com>
>> CC: Kent Baxley <kent.baxley@canonical.com>
>> CC: stable@vger.kernel.org [v3.14+]
>
> Imo this part is a bit frisky - if we get positive testing on the xps13
> machines I'd shovel this into 3.15-fixes and then resurrect the old hack
> for stable kernels. Imo this is risky enough that we should attempt to
> backport it before 3.15 is actually released.

I pushed the patches to -fixes per the test reports, and dropped cc:
stable. IOW, this will cover 3.15 and beyond, but we'll still need to do
something about 3.14, the only affected stable. Either way, it'll have
to wait until these get merged to Linus' tree.

Thanks for the review and testing.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center